The reason why you might get a null as a like count is that you came across a post which has the can_like or can_comment fields set to false. These posts cannot be liked or commented because they are present outside of the page (i.e. on a user timeline or on particular website) and their owner didn't allow public likes or comments (even if publicly visible). If you are going to find such activities in the stream of the page is because they tagged the page in their message or photo.

Edit in answer to your first comment.

Question: why doesn't the following query give back the posts from the owner of the page: